This invention relates to prop-fans and particularly to a low noise prop-fan and the method of designing a planform for achieving a low noise profile.
For purposes of this disclosure, a prop-fan is of the type being developed by the Hamilton Standard Division of United Technologies Corporation, the common assignee and which is described in American Institute of Aeronautics and Astronautics Paper No. 75-1208 presented at the Propulsion Conference held on Sept. 29-Oct. 1, 1975 at Annaheim, California incorporated herein by reference. For the sake of convenience and simplicity, the prop-fan, as considered herein, is a relatively small diameter, highly loaded multiple bladed (more than four blades) variable pitch propulsor having swept blades with thin airfoil section in the lower portion toward the root that has a solidity factor of 1.00 or greater and the airfoil section closer to the tip has a solidity factor of less than 1.00.
As is well known, there has been a recent concern of ecology and noise pollution and aircraft have been under attack for its noise contribution. Needless to say, a great effort has been and is currently in progress to reduce the noise level in the environment while concomitantly enhancing of the conservation of energy. Accordingly, this invention is directed to reducing the noise level of aircraft propulsors while improving or at least not impairing their fuel economy.
The prop-fan contemplated in this invention is a small diameter propeller, say, nearly half the diameter of the conventional propeller and having say from six to twelve blades. The rotational speed is such that the airfoils of the propeller blades are operating at or above a critical Mach No. and the tip may be operating at transonic and supersonic speeds.
Obviously, without any consideration to the noise level and based on current theory and state-of-the-art design, the noise propagated at this speed would realistically be considered unacceptable. Attempts have been made to design the sweep distribution utilizing aerodynamic theories developed for wings of aircraft in the belief that inasmuch as blades are airfoils, the same theory should be equally valid. Hence, with this in mind the first attempt to design the sweep of the blade was to utilize the wing theory--known as the cosine law for ascertaining the Mach surfaces.
Although this was considered by some skilled in the art to be a logical approach for designing the planform of the blade, it was soon discovered that blades made according to this theory did not produce the results contemplated.
Accordingly, and according to this invention, it was ascertained that in order to achieve a noise reduction, it was necessary to design the sweep distribution of the blades so that the leading and trailing edges fall behind the leading and trailing Mach surfaces, respectively. To achieve this end, the planform shape is first arrived at mathematically from well known linearized or non-linearized aeroacoustic theory as say from that theory prescribed in the AIAA Paper No. 76-565 presented at the Aero-Acoustics Conference held on July 20-23, 1976 at Palo Alto, California incorporated herein by reference. This establishes a trial planform from which the sweep can then be tailored by actual trial and error to meet the results desired. In actuality the sweep is increased to the point where satisfactory noise reduction is achieved.